Therefore, the U.S. Pat. No. 4,770,179 which describes an oximeter can be cited. This oximeter uses a first red LED diode and a second infrared LED diode in order to irradiate the blood vessels of a person and a photoreceptor, such as a photodiode, for sensing the reflected light. With this oximeter, a large error source of the optical signal by the modulation of said signal during movements of the person is observed. This modulation can be greater than the modulation of the optical absorption by the blood by a few orders of magnitude, which constitutes a disadvantage of such an oximeter. For this reason it does not make it possible to take into account the movements of the person in order to determine precisely the modulation of the volume of the blood vessels, i.e. the heartbeat.
In the same connection, the patent FR 2 511 594 which describes a device for controlling the pulse can be cited. This device comprises two infrared LED diodes, and a photodetector for measuring the cardiac pulsation of a sportsman when moving for example. However, precise differentiation between the pulse measurement of a person who is moving and a person in a stationary position is not provided. There is likewise not taken into account the fact that the absorption by the blood is more selective in wavelength than the modulation of the signal by the movements of the person in order to make it possible to effect the pulse measurement. Furthermore, equally nothing is provided for suppressing the effect of ambient light which is another source of error for the pulse measurement.
One solution to the problem of the pulse measurement connected to the movement of the person is proposed in the patent application US 2005/0075553. This patent application describes a method and a portable device, such as a wristwatch, for managing biological information, such as heartbeat and functions of the nervous system. The wristwatch comprises a sensor device having an LED diode of a green colour, an infrared LED diode and a photodiode for sensing the light reflected by the skin and the blood vessels and which originates from the LED diodes which are switched on alternately, and a microcontroller unit connected to the sensor device in order to control it. The green light waves make it possible to obtain a signal for detecting the pulse beat and movement whilst the infrared light waves make it possible essentially to obtain a signal from the movement of the person wearing the watch.
The microcontroller unit receives digital signals from an analogue/digital converter of the sensor device. This unit effects a fast Fourier transformation (FFT) of the digital signals received from the green and infrared light waves. After standardisation of the Fourier spectra, the spectrum of the infrared light waves is subtracted from the spectrum of the green light waves in order to be able to remove the effect of the movement and to calculate the pulse of the person. However, the intensity of the light waves must be well adapted in order to allow complete removal of the effect of the movement after the operation of subtracting the Fourier spectra of the light waves, which is a disadvantage. Furthermore, high electrical consumption is observed in effecting the pulse measurement which constitutes another disadvantage.